Also Thursday, a six-month ban on sales by shareholders who own more than 5 percent of a company was due to expire. Regulators announced this week that to avoid fueling further volatility, such sales will be limited to private transactions.

The Shanghai benchmark more than doubled between late 2014 and its June 12 peak as millions of novice investors bought shares.

Chinese leaders had encouraged the public to buy in hopes of raising money to overhaul state industry. The market rout alienated small investors who were left holding shares worth less than they paid.

Authorities say shares bought by state companies will be transferred to China's sovereign wealth fund to avoid depressing prices by selling them in the open market. The ban on new IPOs was lifted in November.